Show filament subtypes in all mapping dropdowns (#624)

  All filament mapping dropdowns (single-printer, multi-printer, and
  "Print to Any" model-based) showed only the base material type (e.g.
  "PLA") without distinguishing subtypes like "PLA Basic" vs "PLA Matte",
  making entries with different subtypes but same color look identical.

  Backend: Add tray_sub_brands to available-filaments response and include
  it in the dedup key so different subtypes of the same color appear as
  separate entries.

  Frontend: Show tray_sub_brands in FilamentMapping, FilamentOverride, and
  PrinterSelector dropdowns, falling back to base type when unset. Add
  traySubBrands field to LoadedFilament interface.